The work proposed in this application is divided into two separate but closely related projects. This first involves a careful examination of active and passive volume regulatory phenomena exhibited by normal human RBC whose ion and water content has been perturbed under a variety of conditions. We shall measure the net movement of cations, anions, and water as well as the unidirectional fluxes of Na, K, and Ca. We plan to use various inhibitors as probes of these volume regulatory processes. We will study the role of the co- and counter-ions and we shall search for a plasma factor which might facilitate the volume regulatory process. Our studies will be performed under rigorously controlled conditions of pH and osmolality. The second project will attempt to delineate the role of altered cell volume homeostasis in the pathophysiology of sickle cell disease. We will focus particularly upon the changes in cell ion and water content brought about by both acute and chronic deoxygenation. We shall examine the Ca dependence of this process and we will observe whether agents such as quinidine and cetiedil, known to inhibt Ca-dependent K leak (the Gardos phenomenon), are able to prevent the changes in ion and water conent brought about by sickling. Finally, we ahsll focus on the development of therapeutic modalities whose locus of action is at the membrane level. The goal of such treatment programs would be to lower the intracellular concentration of hemoglobin S and thus reduce its tendency for aggregation.